There are hitherto proposed a large number of compounds as traction drive fluids. For example, proposed in U.S. Pat. No. 3,652,418, etc., are decalin, perhydroanthracene, polycyclohexyl compounds, bicyclohexyl compounds, dicyclohexyl compounds, hydrogenation products of .alpha.-methylstyrene dimers, adamantanes, alkylbenzenes and hydrogenation products of styrenated cumene.
With the development in automobile technology in recent years, the sizes of the above-mentioned traction drive devices have been reduced, while they are used under severer conditions of higher speed and higher load. Accordingly, the use conditions for the traction drive fluids in these devices have become severer at higher temperatures.
Among the foregoing hitherto proposed compounds, however, those put into practice are not many because of their impractical traction coefficients and the difficulty of obtaining the raw materials for industrial-scale production. Only 2,4-dicyclohexyl-2-methylpentane proposed in U.S. Pat. No. 3,994,816 can be exemplified as a material that is acceptable for practical uses.
In the traction drive devices, as power is transmitted by the shearing stress caused by an oil film of a traction drive fluid that is formed between rotating bodies (revolving bodies) which are rotating in relative relationship, the thickness of the film must be maintained to a certain value. Accordingly, a viscosity to a certain level is required even in high temperature conditions. By this fact, high viscosity fluids are required recently. In addition, not only the viscosity but also a high traction coefficient is also necessary, of course.
Meanwhile, even in the case of the typical compound of 2,4-dicyclohexyl-2-methylpentane among those barely put into practice, the viscosity is not always satisfactory though the traction coefficient is high to some extent. In other words, the viscosity (at 100.degree. C.) of this compound itself is only 3.6 cSt (10.sup.-2 cm.sup.2 /sec). Therefore, it was proposed to add a viscosity index improving agent such as polymethacrylate and polyisobutylene in order to raise the viscosity. These viscosity index improving agent can raise the viscosity indeed, however, the traction coefficient is lowered to make the matter worse. In addition, when a traction drive fluid is used for a long period of time under severe conditions, undesirable results are often caused to occur that the viscosity is lowered due to the deterioration by the addition of these additives. Because the traction drive fluid is subjected to quite severe conditions in view of oxidation reaction, the requirement with regard to oxidation stability is also very severe.
Incidentally, various lubricants have been proposed and put into practice for the use in rotating members such as rotary bearings in which parts are rotated freely and substantially independently. For example, a mixture of hydrogenated linear dimer of .alpha.-methylstyrene and hydrogenated linear trimer of the same is proposed In U.S. Pat. No. 3,925,217. However, what is called lubricant oil improves the sliding among rotating members to allow them to rotate freely or separately, which fact contrasts with the function of the traction drive fluid. In other words, the lubricant oil is used to assure the free or independent movement, which is quite different from the functions and effects of the traction drive fluid according to the present invention. Therefore, it is not possiible at all to suppose the function and effect in power transmission of the present invention from the function and effect of such a lubricating oil. In other words, it is general to consider that the so-called lubricant oil used for rotary bearing members to allow them to rotate freely and independently, is not suitable for use as a traction drive fluid. Accordingly, those skilled in the art cannot suppose that the lubricant oil disclosed in the foregoing patent specification is useful as a traction drive fluid.
Furthermore, it is disclosed in U.S. Pat. Nos. 3,595,796 and 3,598,740 that the trimers of .alpha.-methylstyrene is used as a traction drive fluid. However, all the oligomers disclosed in these patent specifications are what is called cyclic compounds of hydroindane type or compositions mainly containing these cyclic compounds. The present inventors have made detailed investigation in view of the foregoing severe standards required in recent years in connection with traction drive fluids. As a result, it was found that the cyclic compounds disclosed in the above patent specification, especially cyclic trimers and cyclic tetramers, have low oxidation stability and they cannot meet the recent severe requirement level.